In many cases it is desirable to determine the degree of filling of a vessl with a liquid and especially the volume of liquid contained in a receptacle. In many cases, the volume of liquid or the degree of filling in a vessel is determined by detecting the position of the level of liquid, i.e. the liquid-gas interface in the vessel.
In systems for determining the residual volume of a fuel in the fuel tank of a vehicle, for example, a float or like level sensor can be provided to signal the degree of filling of the reservoir to the vehicle operator, the degree of filling being registered on a fuel gauge on the dashboard of the vehicle, for example.
In this system, the level sensor is an analog signal generator producing a signal whose amplitude directly represents the degree of filling of the fuel tank and which is connected electrically to the gauge which provides an analog output, i.e. a needle swing of a galvanometer or the like whose angular displacement is proportional to the signal.
In practice this method of displaying the degree of filling of a fuel tank is unsatisfactory because the liquid level and the gauge reading depend upon a variety of factors which represent perturbations of the actual measurement of the residual fuel. For example, the system is sensitive to the inclination of the tank, the sensor and the housing of the automobile, to vibration, to vehicle acceleration and deceleration. The sensor is usually a potentiometer whose resistence value is varied by the float.
Because of these disturbances which can readily affect the measurement, efforts have been made to find other methods and devices which can be effective for the purpose described. Investigations have resulted in piezoelectric, thermal and capacitive devices, some of which are also sensitive to vibrations in the level brought about by different inclinations of the automobile.
For example, in the capacitive approach, two plates are spacedly juxtaposed to form an electrical capacitor and can be disposed vertically in the fuel tank. The fuel in the tank is a dielectric liquid and hence a fluid whose dielectric constant differs from that of the gas (air or vapor) above the liquid-gas interface.
Thus, as the liquid level rises or falls in the tank and the gap between the plates becomes more or less occupied with the dielectric liquid, the electrical capacitance of the device changes and this capacitance can be transformed into an output signal by appropriate circuit means to signal the liquid level. Such devices are sensitive not only to the inclination of the automobile but also to temperature and the quality of the fuel since both temperature and composition of the fuel affect the dielectric constant.
To eliminate these effects, a reference capacitance whose gap is completely flooded by the fuel must be used and the capacitors may require a bridge circuit as part of the sensor. This makes the sensor and the entire device relatively costly without significantly eliminating the problem with respect to variation with inclination. While cost generally does not enter as a significant factor for fuel gauges for aircraft purposes, it is a significant factor for automobiles.
These points have been made slowly to demonstrate that there are numerous level-sensing devices available which have various drawbacks and which can be more or less costly depending upon the purposes to which such devices are put.
In Marine Week, Vol. 6 No. 11 (1979), pages 8 and 9, there is described a device for measuring the level of liquid in a chemical reservoir or tank utilizing a device sensitive to the pressure generated by the column of liquid, namely with a transducer having a vibrating wire whose internal frequency of vibration changes as the pressure changes and which is a function of the tension of the wire which depends upon the pressure of the liquid and hence upon the height of the liquid column, the density of the liquid and the temperature.
However, this approach has not found widespread application in fuel gauges for vehicles because it too is sensitive to the inclination of the tank if the height of the column is measured only at a single point. Measurements at more than one point make the system unduly complicated. Consequently, this system does not solve the problems outlined above.
It is also possible to measure the volume of the gas space remaining in a vessel of known capacity which has been partly filled with a liquid, thereby allowing this measured volume to be subtracted from the known volume of the vessel to give the volume of the liquid. In this method, the gas space is in contact with a diaphragm which partly delimits this space, which is displaced and which is permitted to return to its normal position after such displacement. The time required to reestablish the initial pressure in the chamber after the displacement of the diaphragm, utilizing the admission of air through a calibrated orifice is a measure of the volume of the gas space.
This sytem, while not sensitive to inclination of the liquid surface in the vessel, is sensitive to the vapor pressure in the reservoirs, the imprecision and instability of orifice flow, variations in pressure in the tubes or pipes utilized in the system and the like. In addition, the system requires a variety of tubes, pipes, valves, and above all, an expensive calibrated orifice which contributes to the cost of the system and introduces elements which may adversely affect the reliability thereof.
In Japanese patent No. 146 676/79, the volume of gas in a vessel is detected by acoustic means, the vessel being open to a sonic pressure source or field of predetermined frequency with the pressure and frequency being determined proximal to the opening of the vessel and being a function of the volume of gas therein. This system is, of course, limited in utility to open vessels in which the free gas columns forms a resonant system.